1. Field of the Invention
The present invention relates to imaging systems. More specifically, the present invention relates to range Doppler imaging systems.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art:
Range Doppler (RD) imaging systems are active imaging systems in which a target is illuminated by an electromagnetic beam. An image of the target is generated by extracting the differential Doppler shift information contained in the beam reflections from the target. The differential Doppler shift information is indicative of the relative motion or vibration of various target reflecting elements.
Conventional RD imaging systems are operative within the microwave and infrared regions down to wavelengths of approximately 10.6 microns and may be serviced by telescopes of moderate size.
Conventional long range imaging systems, as well as long range RD imaging systems, operating to ranges of 200 km and above require telescopes (or baselines) on the order of tens of meters (for visible radiation) to kilometers (for infrared radiation) to achieve a desired target resolution. (The baseline of an imaging system refers to the diameter of a receiving antenna array or the diameter of the aperture of a receive telescope.)
In addition to the practical difficulties encountered in constructing large baseline systems, both conventional and RD large baseline systems are susceptible to aberrations in the wavefronts of target reflections as received at the telescope when imaging through atmospheric turbulence. In the case of RD systems operating with large apertures, such wavefront aberrations typically destroy the spatial coherence between the target reflection and a local oscillator beam combined therewith prior to illumination of a detector. This lack of spatial coherence produces a weak and strongly fluctuating signal at the output of the detector. Hence, RD imaging systems are prone to sensitivity degradation due to atmospheric turbulence.
Further, the heterodyne detection apparatus typically employed in conventional RD systems requires that the target illuminator have a temporal coherence length on the order of the round trip distance from the illumination source to the target. Unfortunately, such coherence length problems tend to be exacerbated as the operational wavelength of the illumination source is reduced to afford enhanced target resolution. For example, long range RD systems are currently prevented from operating at visible wavelengths due to the unavailability of stable laser illuminators having coherence lengths of 400 km or longer. Further, the requisite laser local oscillator tuning range increases as the illuminating wavelength decreases. Hence, constraints on laser illuminators and local oscillators associated therewith limit the operating frequency spectrum of conventional imaging systems in long range applications.
Despite the obstacles mentioned above inherent in adapting conventional and RD imaging systems to shorter wavelength operation, successful realization of such systems at visible wavelengths would provide significant advantages. For example, many target surfaces which are extended diffuse scatterers at visible and shorter wavelengths do not behave as such in the longer wavelength infrared and microwave regimes. Thus, a larger percentage of a target can be imaged at visible wavelengths. Further, infrared detectors need to be cooled to attain near quantum limited performance while visible detectors do not.
Hence, a need in the art exists for a long range imaging system capable of operating at visible wavelengths.